The present invention relates to mechanical actuators. More particularly, the present invention is applied in conjunction with high force, low travel linear actuators.
Actuators, such as high output force actuators are well known in the art. Some high output force actuators are motor or gear box driven, such as a ball screw actuator with a high ratio motor/gear box drive. Others are pneumatic, hydraulic or thermochemically driven. Thermochemical actuators usually employ a thermally expansible medium or compound, such as a wax, to extend a piston and thereby drive an external device.
Examples of the actuators are disclosed in U.S. Pat. No. 5,025,627 (Schneider). U.S. Pat. No. 5,396,770 (Petot et al.), U.S. Pat. No. 5,020,325 (Henault), U.S. Pat. No. 5,685,149 (Schneider et al.), U.S. Pat. No. 5,738,658 (Maus et al.), U.S. Pat. No. 5,720,169 (Schneider), U.S. Pat. No. 5,419,133 (Schneider), and U.S. Pat. No. 5,222,362 (Maus et al.), the disclosures of which are whereby incorporated by reference in their entirety.
These thermochemical actuators are described with various nomenclature including heat motors, thermochemical actuators, mechanical actuators, electrothermal actuators, high output paraffin actuators (HOP actuators), pneumatic actuators, hydraulic actuators, and the like. All of these devices actuate a shaft in response to heat energy. The heat is applied to a variable volume chamber filled with a working medium such as wax or fluid. The working medium expands, thus expanding the chamber volume and driving the shaft or piston. The motion of the shaft can be used to drive various external devices. These actuators are utilized in various applications including automotive systems and satellites. Wax actuators are used in automobile radiators to open a water circulation valve when the engine reaches operating temperature.
For example, high output paraffin (HOP) actuators are made by Starsys, Inc. in Boulder, Colo. They use paraffin, or wax, as an actuating technique by utilizing the about 15% volume expansion that occurs when paraffin melts. The volume expansion increases the hydrostatic pressure in a pressure housing, applying that pressure to a rubber boot that squeezes an output rod out of the HOP housing.
When using such actuators, it is absolutely essential to have an external means of removing power to stop heating of the paraffin when the actuator stroke is complete or whenever the output rod has reached an immovable external stop. Otherwise, the pressure in the actuator housing will continue to increase, destroying the actuator. However, if the means to remove power does not function properly or fast enough, the actuator may be destroyed. There is a need for a means of absorbing the increase in pressure in the actuator even if the power supply is not removed.
The most commonly used means for heating the paraffin or wax is an electrical heater. Some techniques for removal of power include using a position sensor, such as a microswitch or reed switch, to sense the end of stroke and, either directly or indirectly, interrupt power to the actuator heaters. As soon as power is removed, the actuator output rod starts to retract, the position sensor again applies power to the actuator, and this cycle continues until power is removed from the circuit. A drawback to this technique is that, if the actuator output rod encounters an obstruction, either intentional or unintentional, before it reaches its planned end of stroke, the switch at the end of stroke is not triggered, and the actuator will be destroyed.
Many users of paraffin actuators have, at one time or another, damaged an actuator through inadvertent mishandling while testing various systems. Specifically, if one leaves the power applied to the internal heaters after (a) the output rod has reached the end of its travel or (b) the output rod has met an immovable obstruction such as the end of travel of the adjacent components, the internal pressure in the paraffin chamber increases to a point where internal parts of the actuator fail in accordance with design, preventing external release of the wax. This failure requires the return of the actuator to the manufacturer for repairs at significant cost.
When such actuators are utilized in aircraft or spacecraft, it becomes more important to provide a means for eliminating actuator failure resulting from pressure buildup. Additionally, when such actuators are used in remote applications, such as orbiting satellites, failure of the actuator may result in the loss of the satellite, discharge of the expanding medium on other parts of the satellite, destruction of the piston or actuation rod, or other damage to the satellite system. This damage can occur when the power removal mechanism fails or when external heat is applied other than from the intended power source. Additionally, when used in such remote applications, it is desirable to be able to reuse the actuator after such failure or pressure build-up.
Accordingly, there is a need for a system that would eliminate failure of the actuator due to pressure buildup even if the power supply to the actuator is maintained.
Additionally, there is a need for a system that eliminates failure of the actuator due to pressure buildup, by allowing the piston or actuator rod to travel its full path even in the face of an obstruction.
There is also a need for a thermochemical actuator that is reusable even after excessive pressure triggers a release mechanism.
There is also a need for an actuator safety mechanism for an actuator that can be triggered without any external input or power.